In digital communications systems, the physical and electrical characteristics of the transmission facility normally limits the maximum rate at which data can be transmitted, the maximum transmission distance, the noise susceptibility and the emissions that cause radio frequency interference (RFI). To overcome this limitation, common practice is to reduce the data symbol rate (the baud rate), to a lower frequency by generating codes with multiple levels, multiple phases, or both.
This multilevel, multiphase coding can be very expensive to design and manufacture and can consume a great deal of power, particularly at high speeds. This can prevent its use in cost sensitive applications, such as in computer local area networks.
An alternative technique that reduces the cost and power consumption is to use multiple pairs of wires to reduce the data rate on each pair, thus allowing for an increase in transmission distance for the same performance, and a reduction in RFI. In many installations, users have extra pairs of wires installed, and are willing to use them to reduce the cost of the communications equipment.
Recent proposals for 100 megabit Ethernet networks include using multiple wiring pairs to reduce the bit rate on each pair, to allow transmissions over common in-office twisted pair wiring without violating the U.S. Federal Communication Commission's RFI emissions guidelines and to provide reliable data transport.
Such systems are restricted to particular data protocols specifically designed to accommodate the data rate, and thus cannot carry any or all data streams which use any protocol.